In micro-electronics devices closure of openings formed in a layer is often needed to seal a cavity reachable via these openings to preserve the ambient of this cavity during further processing and use of the device.
Cavities may be formed as isolation trenches between devices such as bipolar transistors or high voltage devices or between interconnects lines. To improve their insulating properties, these trenches are filled with a gas, preferably ambient air, having a low dielectric constant. European patent application EP 1,672,687 discloses a method for forming these isolating trenches, also known as air gaps. This method for forming air gaps in a substrate comprises the steps of patterning a hole in the substrate, partly filling this hole with a sacrificial material, forming spacers on the sidewalls of the unfilled part of the hole to locally narrow the opening, removing at least part of the sacrificial material through the narrowed opening, and sealing the opening by depositing a sealing layer over the narrowed opening and surrounding spacers.
Another area in which cavities may be formed is the field of micro-electromechanical systems (MEMS), which often require an encapsulation under vacuum or under a controlled ambient and pressure in order to ensure either a good performance or an acceptable lifetime of operation. The encapsulation has to be performed without the deposition of sealing material on the MEMS device, which can cause damage to the device.
The most popular approach is based on wafer bonding. Here, the sealing is performed by connecting two wafers (device wafer and capping wafer) together by means of a re-flowable material.
Alternatively, encapsulation can be done by fabrication and sealing of surface micro-machined membranes. The use of conformal LPCVD (low-pressure chemical vapor deposition) films is a known method for encapsulation at low pressure. The sealing of the cavity comprising the MEMS devices is done while depositing the conformal film. Hence, the ambient and pressure of the sealed MEMS device are those of the deposition chamber.
Methods for sealing at higher pressures up to the order of atmospheric pressure and a few times that value, by the deposition of thin films, are however, not widespread.
Moreover, most of these atmospheric pressure techniques do not prevent material deposition inside the cavity. MEMS devices can be very fragile and deposition of material on the device is preferably avoided. Deposition of material on a MEMS device will also change the mass of the device and thus the resonance frequency, which also needs to be avoided.
The openings which are typically produced in the sealing membrane using current approaches have been etched, and are therefore sometimes quite large. Etching openings moreover requires patterning, masking steps, etc. that complicates the whole process of encapsulation. Such openings are provided mostly for allowing a sacrificial material etchant to reach out below the membrane or film, which had been supported temporarily by a sacrificial material, thereby dissolving or removing the sacrificial material and releasing the film, at least locally. The use of such a sacrificial material may be used in producing overhanging structures in packaging and MEMS processing technology.
In EP 1,433,741 a method is described for closing openings in a film, with a reflow material, the method comprising: depositing an intermediate layer on the film, which is thermally stable during reflow to thereby narrow down the opening to be sealed by formation of a collar or shoulder; depositing a reflow or sealing layer on the intermediate layer under a first set of pressure and ambient conditions to further partially close the opening, and reflowing the reflow or sealing layer under a second set of pressure and ambient conditions to close the opening by the reflow layer covering over the opening.
In EP 1,843,971 a method for removing a sacrificial material under a membrane is disclosed, the method comprising depositing a layer of sacrificial material; providing the membrane directly on top of the sacrificial material, making the membrane porous in another processing step such that pores and/or holes are formed which extend from the front surface to the back surface of the membrane, and applying a sacrificial material etchant for removing the sacrificial material until the sacrificial layer is at least partially removed by the sacrificial material etchant through the membrane.
There is a need for alternative method for closure of openings, in particular for zero-level or wafer level packaging solutions, which simplify process sequences, and which reduces the risk of having sealing material penetrating the area reachable through the opening, e.g., as for instance a cavity under the sealing membrane comprising, for instance, a fragile object such as a MEMS device.